Modification Of LiFePO₄/C Cathode Materials And Optimization Of The Electrode Recipes

LiFePO4/C was directly synthesized by the mechanochemical reaction. The LiFePO4/C product demonstrates excellent fast charging and discharging ability at room temperature with the initial discharge capacities of 109.1mA·h/g (5C), 118.7mA·h/g (3C), 126.8mA·h/g (2C) and 140.7mA·h/g (1C), respectively after 1C rate rapid charge. The cycling performances of this material at different temperature were investigated. LiFePO4/C presents excellent cycle stability at room and lower temperature, which remains a discharge capacity of 112mA·h/g at 1C rate after 1000 cycles at 25℃, with capacity fading less than 18%. And at -20℃the initial discharge capacity is 74.8mA·h/g at 1C rate, but there is almost no attenuation after 1000 cycles. At 55℃the initial discharge capacity is 160.6mA·h/g at 1C rate, but the discharge capacity decreases obviously after 200 cycle. The cycle stability at high temperature should be improved for the practical use of LiFePO4/C cathode materials in lithium ion batteries.The influence of fluorine substitution on the properties of LiFePO4/C was studied. LiFe(PO4)1-xF3x/C (x:0.025, 0.05, 0.075, 0.1) samples were fabricated by adding LiF in the starting materials. The electrochemical performances of LiFePO4/C composite increase obviously by fluorine substitution. The LiFe(PO4)0.925F0.225/C sample shows outstanding rate performance and cycling stability at high temperature. Its discharge capacity at different rate are 96.6mA·h/g (30C), 128.1mA·h/g (8C), 135.1mA·h/g (5C) and 147.4mA·h/g (1C), respectively. And the discharge capacity remains 80.9mA·h/g after 1000 cycles at 1C rate. However fluorine substitution do not improve electrochemical performances of LiFePO4/C at low temperature.